Switch from master to main for default branch #1622

It's been a little over a year since GitHub switched from master to main (October 1st 2020). Maybe we can do the same now before releasing 3.0.0.

https://github.blog/changelog/2020-10-01-the-default-branch-for-newly-created-repositories-is-now-main/

GitLab also switched to main as the default branch for new projects in 14.0, so this should make the docs more relevant now.